Forming fabrics for papermaking have undergone significant changes in recent years stemming from a desire to form better quality sheets under conditions of increased operating speed and higher quality standards.
Traditionally, forming fabrics have been woven in a wide variety of single layer constructions, but it was found that such fabrics cannot provide adequate performance under certain operating conditions. For example, when single layer forming fabrics are designed with life expectancy as the chief criteria, coarse meshes and large yarns are used; however, such fabrics are frequently not capable of providing the optimum in terms of sheet formation, fines retention, and reduced sheet mark.
To achieve the best sheet properties, it has been found necessary to employ fabrics with finer mesh sheet contact surfaces. If such fabrics were made in single layer constructions, they would not provide adequate life.
In order to satisfy requirements for extended life as well as enhanced sheet quality, many of today's forming fabrics are manufactured in multiple layers, with large yarns and coarse meshes in the underside of the fabric to provdie wear resistance, and fine mesh, smaller yarns woven into the sheet contact layer of the fabric, to provide for increased fines retention and reduced sheet mark.
In the formation of the paper sheet, it has lately been shown that the furnish, which contains both fines and larger stock particles, does not form up in a smooth and continuous process as the sheet progresses along the forming table; rather, the furnish tends to be agitated during the formation process and as a consequence of the turbulance that occurs, a very significant portion of the fines fraction contained in the furnish is lost during formation. Rapid and turbulent drainage forces encountered during sheet formation cause repeated disturbances to the precoat which forms upon the forming fabric mesh, allowing for the repeated escape of fines during the early part of the drainage cycle.
Although the idea of using multiple layer forming fabrics has provided an improvement in sheet quality and fines retention, the cost of such fabric compared to single layer fabric is substantially greater owing to the added weaving time, the need for more expensive finer yarns, and the greater difficulty in achieving sufficient product perfection to satisfy papermaking needs.
Papermaking machines are being operated at much higher speeds today than they were in the recent past, placing severe demands upon the forming fabrics.
Despite the present trend toward multiple layer forming fabrics with larger yarns in contact with the machine side wear inducing elements, increased resistance to wear, even for such double and triple layer fabrics, is an objective of fabric makers called upon to provide the best service life possible for their customers.
In the case if single layer fabrics, the need to provide wear resistance as well as optimum sheet formation properties must be met in a single layer weave; however, substituting larger yarns automatically results in a coarser mesh fabric which will not form the same quality sheet that could be made on a finer mesh fabric.
Consequently, it is an object of this invention to provide a wear resistant coating to the machine contact side of double and triple layer fabrics to increase their resistance to wear.
It is a further object of this invention to provide a wear resistant coating to the machine contact side of single layer fabrics to increase their wear resistance or to permit the use of finer yarns with increased service life.
It is an object of this invention to provide a simpler, less expensive means to enhance the fines retention efficiency of single layer forming fabrics.
It is also an objective of this invention to improve still further the sheet formation and fines retaining ability of multiple layer forming fabrics.
During the formation of paper on forming fabrics, the furnish undergoes a redistribution through agitation and turbulence, with some agglomeration taking place as stock moves about and flows without restriction toward the nearest aperature in the forming fabric, taking fine particles of stock along with the flow. If some means could be found to restrain this free flow of the fines within the furnish liquid, sheet formation would be improved, and the sheet would possess more uniform properties from one side to the other.
It is an object of this invention to provide a forming fabric with enhanced particle arresting means in its sheet contacting surface so that a higher quality of sheet can be produced having improved fiber uniformity and reduced sheet two-sidedness.
It is an object of this invention to provide forming fabrics with useful sheet contact side appurtanences for the purposes of arresting the motion of fines and paper stock ingredients within the furnish on top of the forming media at the earliest possible opportunity to enhance sheet uniformity.
A common characteristic of paper is the presence of a wire mark caused by uneven movement of fibers on the forming wire, by indentation of sheet fibers into the interstices of the forming fabric, as well as by other means.
It is a still further objective of this invention to reduce the wire mark imparted to the sheet by the forming fabric.